Solar Cell and Manufacturing Method Thereof

ABSTRACT

A solar cell includes a photoelectric conversion substrate, a first electrode on one surface of the substrate, a second electrode on the other surface of the substrate, and a third electrode on the other surface of the substrate. The third electrode extracts electric power from the second electrode, and overlaps the second electrode at the periphery in the in-plane direction of the photoelectric conversion substrate. The thickness of the second electrode is larger than that of the third electrode, and the difference between the thickness of the second electrode and that of the third electrode is not less than 10 micrometers and not more than 30 micrometers.

TECHNICAL FIELD

The present invention relates to a solar cell and manufacturing methodthereof, and more specifically relates to a solar cell and manufacturingmethod thereof in which separation of electrodes is prevented.

BACKGROUND ART

Photovoltaic power generation is a clean method of generating electricpower using inexhaustible light energy without discharging toxicsubstances. A solar cell is used for the photovoltaic power generation,which is a photoelectric converter that generates electric power byconverting light energy from the sun into electric energy.

Conventionally, an electrode on the back of a light receiving surface ofa generally produced solar cell is formed by screen-printing silverpaste and aluminum paste on the back surface of a silicon substrate,then drying and firing the pastes. The aluminum formed substantially allover the back surface of the silicon substrate serves as a positiveelectrode. However, in the process of producing a solar cell module, alead tab for extracting electric power cannot be soldered directly tothe aluminum electrode formed of aluminum. Therefore, a silver electrodeis formed, as an electrode for extracting electric power, in such amanner as to partially overlap the aluminum electrode on the backsurface of the silicon substrate (see Patent Documents 1 and 2).

Patent Document 1: Japanese Patent Publication No. 2003-273378

Patent Document 2: Japanese Patent Publication No. HEI10-335267

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

As just described, on the back surface of the substrate of the solarcell, an aluminum electrode for higher electric power output and asilver electrode for extracting the electric power are partiallyoverlapped. In the area where the aluminum electrode and the silverelectrode are overlapped, three metals of silicon in the siliconsubstrate, aluminum in the aluminum electrode, and silver in the silverelectrode are partially alloyed.

However, the overlapped area (alloyed area) is very fragile due tostress assumedly caused by different rate of thermal expansion of eachmaterial that occurs during rapid heating and cooling in firing.Therefore, after the firing for forming an electrode, when, for example,the silver electrode overlaps on the aluminum electrode, the aluminumelectrode with the silver electrode can be peeled off in the overlaparea. Accordingly, the wire cannot be properly soldered in a next modulemanufacturing process.

The present invention was made in view of the problems described above,and it is an object of the present invention to provide a solar cell anda manufacturing method thereof in which the separation of electrodes iseffectively prevented.

MEANS FOR SOLVING PROBLEM

To solve the problems described above and achieve the object, the solarcell according to the present invention includes a photoelectricconversion layer, a first electrode provided on one surface of thephotoelectric conversion layer, a second electrode provided on the othersurface of the photoelectric conversion layer, a third electrodeprovided on the other surface of the photoelectric conversion layer withits periphery overlapping the second electrode in the in-plane directionof the photoelectric conversion layer for extracting electric power fromthe second electrode. The thickness of the second electrode is largerthan that of the third electrode, and a difference in thickness betweenthe second electrode and the third electrode is within a range fromequal to or more than 10 micrometers to equal to or more than 30micrometers.

EFFECT OF THE INVENTION

The solar cell according to the present invention includes aphotoelectric conversion substrate, a first electrode provided on asurface of the photoelectric conversion substrate, a second electrodeprovided on the other surface of the photoelectric conversion substrate,a third electrode provided on the other surface of the photoelectricconversion substrate with its periphery overlapping the second electrodein the in-plane direction of the photoelectric conversion substrate forextracting an electric power from the second electrode. The thickness ofthe second electrode is larger than that of the third electrode, and adifference in thickness between the second electrode and the thirdelectrode is within a range from equal to or more than 10 micrometers toequal to or more than 30 micrometers. Thereby, bonding strength isincreased at an interface between the photoelectric conversion substrateand the second electrode partially alloyed with the third electrode, andbetter bonding can be realized. As a result, the electrode separation(alloy separation) can be effectively prevented. Further, it becomespossible to effectively prevent faulty electrode fabrication caused bythe difference in thickness between the second electrode and the thirdelectrode.

Therefore, according to the present invention, it becomes possible torealize a solar cell in which, while faulty electrode fabrication isprevented, the photoelectric conversion substrate and the secondelectrode partially alloyed with the third electrode are firmly bondedtogether, which effectively prevents the electrode separation (alloyseparation).

Furthermore, in a module manufacturing process after producing the solarcell, a lead tab can be properly welded to an electrode, which reducesfaulty tab welding and improves production yield.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1-1 is a cross section for explaining a general configuration of asolar cell according to a first embodiment of the present invention.

FIG. 1-2 is a plan view for explaining a general configuration of afront surface (a light receiving surface) of the solar cell according tothe first embodiment of the present invention.

FIG. 1-3 is a plan view for explaining a general configuration of a backsurface (a surface opposite to the light receiving surface) of the solarcell according to the first embodiment of the present invention.

FIG. 1-4 is an enlarged schematic of an alloyed area where three metalsof silicon, aluminum, and silver are partially alloyed on the solar cellaccording to the first embodiment of the present invention.

FIG. 2 is an enlarged cross section of a surrounding area of an area B′and an area C′, where the aluminum electrode and the back-surface silverelectrode are partially overlapped on the back surface of a conventionalsolar batter cell.

FIG. 3 is a plot for explaining correlation of a difference in thicknessbetween an aluminum electrode and a back-surface silver electrode afterfiring (a value obtained by subtracting thickness of the back-surfacesilver electrode from thickness of the aluminum electrode) andoccurrence frequency of an electrode (alloy) separation.

FIG. 4 is a plot for explaining correlation of a difference in thicknessbetween an aluminum electrode and a back-surface silver electrode afterfiring (a value obtained by subtracting thickness of the back-surfacesilver electrode from thickness of the aluminum electrode) and frequencyof printing failure.

FIG. 5-1 is a cross section for explaining a method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-2 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-3 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-4 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-5 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-6 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-7 is a plan view for explaining an example of a screen mask usedfor printing with a silver paste in manufacturing the solar cellaccording to the first embodiment of the present invention.

FIG. 5-8 is a cross section for explaining an example of a screen maskused for printing with a silver paste in manufacturing the solar cellaccording to the first embodiment of the present invention.

FIG. 5-9 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 5-10 is a cross section for explaining the method for manufacturingthe solar cell according to the first embodiment of the presentinvention.

FIG. 6 is a cross section for explaining a general configuration of asolar cell according to a second embodiment of the present invention.

FIG. 7 is a cross section for explaining a general configuration ofanother solar cell according to the second embodiment of the presentinvention.

EXPLANATIONS OF LETTERS OR NUMERALS

Semiconductor layer

-   -   11 Silicon substrate    -   13 n-type diffusion layer    -   13 a n-type diffusion layer    -   14 p+ layer    -   15 Antireflective coating    -   17 Aluminum electrode    -   17 a Aluminum paste layer    -   19 Back-surface silver electrode    -   19 a Silver paste layer    -   21 Front-surface silver electrode    -   21 a Silver paste layer    -   23 Alloyed area    -   25 Mesh    -   27 Emulsion    -   29 Mask frame    -   31 Overlapped area of aluminum electrode and back-surface silver        electrode    -   33 Overlapped area of aluminum electrode and back-surface silver        electrode with neighboring area

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Exemplary embodiments of a solar cell according to the present inventionare explained below in detail while referring to the accompanyingdrawings. It should be noted that the present invention is not limitedby the following description, but can be changed in various mannerswithin the scope of the present invention. In the accompanying drawings,scale sizes may vary among the drawings and among members depictedtherein for better understanding.

First Embodiment

FIGS. 1-1 to 1-3 are drawings for explaining a solar cell according to afirst embodiment of the present invention, and FIG. 1-1 is a crosssection for explaining a general configuration of the solar cellaccording to the first embodiment. FIG. 1-2 is a plan view forexplaining a general configuration of a front surface (a light receivingsurface) of the solar cell according to the first embodiment, and FIG.1-3 is a plan view for explaining a general configuration of a backsurface (a surface opposite to the light receiving surface) of the solarcell according to the first embodiment. Incidentally, FIG. 1-1 is across section taken along the line A-A of FIG. 1-3.

The solar cell according to the embodiment includes, as shown in FIGS.1-1 to 1-3, a semiconductor layer 10 that is a photoelectric conversionlayer including a p-type layer 11 that is a p-type silicon substrate asa semiconductor substrate, an n-type diffusion layer 13 with aconductivity type inverse to that of the surface of the p-type layer 11,and a p+ layer (back surface field (BSF) layer) 14 containing a highconcentration of impurity; an antireflective coating 15 provided on alight receiving surface of the semiconductor layer 10 to prevent thereflection of incident light; a front-surface silver electrode 21 thatis a light receiving surface electrode provided on the light receivingsurface of the semiconductor layer 10 substantially in the shape of astick; an aluminum electrode 17 that is a back-surface electrodeprovided substantially all over the back surface of the semiconductorlayer 10 to extract electric power and reflect the incident light; and aback-surface silver electrode 19 that is an electrode to extract theelectric power from the aluminum electrode 17.

In the solar cell according to the embodiment configured as above, whensunlight irradiates the side of the light receiving surface (the side ofthe antireflective coating 15) of the solar cell and reaches a p-njunction surface (a junction surface of the p-type layer 11 and then-type diffusion layer 13) inside, a hole and electron pair on the p-njunction surface is separated. The separated electron moves toward then-type diffusion layer 13. On the other hand, the separated hole movestoward the p+ layer 14. This produces an electrical potential differencebetween the n-type diffusion layer 13 and the p+ layer 14 so that the p+layer 14 has a higher potential. This makes the front-surface silverelectrode 21 connected to the n-type diffusion layer 13 a negativeelectrode and the aluminum electrode 17 connected to the p+ layer 14 apositive electrode so that the electricity flows through an externalcircuit (not shown).

Next, features of the solar cell according to the embodiment areexplained. As shown in FIG. 1-4, in the solar cell according to theembodiment, the aluminum electrode 17 and the back-surface silverelectrode 19 are partially overlapped on the p+ layer 14. FIG. 1-4 is aschematic that depicts an enlargement of an area surrounding theback-surface silver electrode 19 shown in FIG. 1-1, i.e., a crosssection of the surrounding area of an area B and an area C, where thealuminum electrode 17 and the back-surface silver electrode 19 arepartially overlapped on the back surface of the solar cell.

In the area B and the area C where the aluminum electrode 17 and theback-surface silver electrode 19 are partially overlapped, three metalsof silicon in the p+ layer 14 of the silicon substrate, aluminum in thealuminum electrode 17, and silver in the back-surface silver electrode19 are partially alloyed to form an alloyed area 23 as shown in FIG.1-4. While borders of the metals (the silicon, the aluminum, and thesilver) in the area B and the area C are clearly defined in FIGS. 1-1and 1-5, it is needless to say that the areas are partially alloyed andthe borders are actually not clear.

The solar cell according to the embodiment is configured, as shown inFIG. 1-4, such that the periphery of the back-surface silver electrode19 overlaps the aluminum electrode 17 in the in-plane direction of thesemiconductor layer 10. The thickness t_(Al) of the aluminum electrode17 is larger than the thickness t_(Ag) of the back-surface silverelectrode 19, and the difference in thickness between the aluminumelectrode 17 and the backside silver electrode is equal to or more than10 micrometers and equal to or less than 30 micrometers.

With the above configuration, in the solar cell according to theembodiment, the alloyed area 23 is assuredly formed in the area B andthe area C in which the back-surface silver electrode 19 and thealuminum electrode 17 are partially overlapped each other, the aluminumelectrode 17 and the back-surface silver electrode 19 in the alloyedarea 23 are firmly bonded together, and the aluminum electrode 17 andthe back-surface silver electrode 19 are firmly bonded to the p⁺ layer14 of the silicon substrate as shown in FIG. 1-4.

The conventional solar cell is configured, as shown in FIG. 2, such thatthe periphery of the back-surface silver electrode 19 overlaps thealuminum electrode 17 in the in-plane direction of the semiconductorlayer 10. The thickness of the aluminum electrode 17 is larger than thatof the back-surface silver electrode 19. That is, the conventional solarcell, similar to the solar cell according to the embodiment, includes anarea B′ and an area C′ in which the back-surface silver electrode 19 andthe aluminum electrode 17 are partially overlapped each other. In thearea B′ and the area C′ in which the back-surface silver electrode 19and the aluminum electrode 17 are partially overlapped each other, threemetals of silicon in the p⁺ layer 14 of the silicon substrate, aluminumin the aluminum electrode 17, and sliver in the back-surface silverelectrode 19 are partially alloyed.

In the area B′ shown in FIG. 2, the three metals of the silicon in thep⁺ layer 14 of the silicon substrate, the aluminum in the aluminumelectrode 17, and the sliver in the back-surface silver electrode 19 arepartially alloyed, and the partially alloyed aluminum electrode 17 andthe back-surface silver electrode 19 are not separated (alloyseparation). On the other hand, in the area C′, the three metals of thesilicon in the p⁺ layer 14 of the silicon substrate, the aluminum in thealuminum electrode 17, and the sliver in the back-surface silverelectrode 19 are partially alloyed, and the partially alloyed aluminumelectrode 17 and the back-surface silver electrode 19 are separated(alloy separation). In FIG. 2, because of convenience in the drawing,borders of each metal (silicon, aluminum, and silver) are clearlydefined; however, it is needless to say that the areas are partiallyalloyed and the borders are actually not clear.

In the conventional solar cell, however, the partially overlapped area(partially alloyed area) is very fragile and its bonding capability isreduced due to stress assumedly caused by a different rate ofthermal-expansion of each metal rapidly heated and cooled during firingin manufacturing. Therefore, after the firing for forming an electrode,the aluminum electrode 17 with the back-surface silver electrode 19 canbe separated in the overlapped area as shown in the area C′ in FIG. 2.In this case, there occurs a problem that a lead tab cannot be properlywelded to the electrode in a next module manufacturing process.

According to a research by the inventors, in the observation of a solarcell after the firing of an electrode in fabrication of the cell, it hasbeen found that an electrode separation (alloy separation), or aseparation of the aluminum electrode 17 and the back-surface silverelectrode 19 from the silicon substrate (p⁺ layer 14), sometimes occursin the overlapped area of the aluminum electrode 17 and the back-surfacesilver electrode 19 (see area C′ in FIG. 2). Besides, it has been foundthat the separation of the aluminum electrode 17 and the back-surfacesilver electrode 19 from the silicon substrate (p⁺ layer 14) is onefactor for defective welding of the lead tab to the electrode in themodule manufacturing process. It has also been found by the observationthat the electrode separation (alloy separation) occurs at the interfacebetween the silicon substrate and the electrodes (where aluminum andsilver are partially alloyed) beneath the periphery of the overlappedarea of the aluminum electrode 17 and the back-surface silver electrode19 as shown in the area C′ in FIG. 2.

Then, as a result of further research by the inventors, in the solarcell configured that the periphery of the back-surface silver electrode19 overlaps the aluminum electrode 17 in the in-plane direction of thesemiconductor layer 10, when the thickness t_(Al) of the aluminumelectrode 17 is larger than the thickness t_(Ag) of the back-surfacesilver electrode 19 and a difference between the thickness t_(Al) of thealuminum electrode 17 and the thickness t_(Ag) of the back-surfacesilver electrode 19 is in a range from not less than 10 micrometers tonot more than 30 micrometers, it becomes possible to achieve a solarcell in which the silicon substrate (p⁺ layer 14) is firmly bonded tothe aluminum electrode 17 (partially alloyed with the back-surfacesilver electrode 19) and the electrode separation (alloy separation) isprevented from occurring.

FIGS. 3 and 4 are graphs of characteristics obtained from the experimentin which the thickness of the aluminum electrode 17 and the back-surfacesilver electrode 19 was varied in the solar cell configured such thatthe periphery of the back-surface silver electrode 19 overlaps thealuminum electrode 17 in the in-plane direction of the semiconductorlayer 10. FIG. 3 is a plot of correlation between the difference in thethickness of the aluminum electrode 17 and the back-surface silverelectrode 19 after the firing in the electrode forming process (a valueobtained by subtracting the thickness t_(Ag) of the back-surface silverelectrode 19 from the thickness t_(Al) of the aluminum electrode 17),and an occurrence frequency of the alloy separation. The occurrencefrequency of the alloy separation indicates how many cells causes thealloy separation among the tested cells.

On the other hand, FIG. 4 is a plot of correlation between thedifference in the thickness of the aluminum electrode 17 and theback-surface silver electrode 19 after the firing in the electrodeforming process (a value obtained by subtracting the thickness t_(Ag) ofthe back-surface silver electrode 19 from the thickness t_(Al) of thealuminum electrode 17), and frequency of printing failure. The printingfailure includes insufficient paste-coating, such as blurred printing,occurred in an alloyed area and around the alloyed area, and faultyelectrode fabrication. The frequency of the printing failure indicateshow many cells causes the printing failure among the tested cells.

It can be seen in FIG. 3 that, when the difference in the thickness ofthe aluminum electrode 17 and the back-surface silver electrode 19 afterthe firing in the electrode forming process (a value obtained bysubtracting the thickness t_(Ag) of the back-surface silver electrode 19from the thickness t_(Al) of the aluminum electrode 17) is equal to ormore than 10 micrometers, the frequency of the alloy separation sharplydecreases. On the other hand, it can be seen in FIG. 4 that, when thedifference in the thickness of the aluminum electrode 17 and theback-surface silver electrode 19 after the firing in the electrodeforming process (a value obtained by subtracting the thickness t_(Ag) ofthe back-surface silver electrode 19 from the thickness t_(Al) of thealuminum electrode 17) is equal to or more than 30 micrometers, thefrequency of the printing failure sharply increases.

According to another experiment and test, it has been found that alloyseparation occurs at the interface between the silicon substrate and theelectrodes (where aluminum and silver are partially) beneath the end ofthe overlapped area of the aluminum electrode 17 and the back-surfacesilver electrode 19. It has also been found that, when silverconcentration at the interface is higher, the electrode separation(alloy separation) is likely to occur.

In an area where the aluminum or the silver directly contacts thesilicon, the electrode separation (alloy separation) is not detected andthey are bonded well. Therefore, it can be presumed that, when an alloyof aluminum and silicon contains silver at a certain level, the bondingcapability is reduced due to a change of a thermal expansion factor orother actions.

According to the above result, to prevent the electrode separation(alloy separation) from occurring, it is necessary to reduce the silverconcentration around the interface between the silicon substrate (p⁺layer 14) and the electrodes (alloy). In addition, it is effective toincrease the thickness t_(Al) of the aluminum electrode 17 or to reducethe thickness t_(Ag) of the back-surface silver electrode 19. On theother hand, the printing failure is due to an inadequate coverage of thepaste caused by the difference between the thickness t_(Al) of thealuminum electrode 17 and the thickness t_(Ag) of the back-surfacesilver electrode 19.

Thus, by setting the thickness t_(Al) of the aluminum electrode 17larger than the thickness t_(Ag) of the back-surface silver electrode 19as well as setting the difference between the thickness t_(Al) of thealuminum electrode 17 and the thickness t_(Ag) of the back-surfacesilver electrode 19 not less than 10 micrometers and not more than 30micrometers, the inventors has achieved a solar cell with theconfiguration that the periphery of the back-surface silver electrode 19overlaps the aluminum electrode 17 in the in-plane direction of thesemiconductor layer 10, in which the silicon substrate (p⁺ layer 14) andthe aluminum electrode 17 (partially alloyed with the back-surfacesilver electrode 19) are firmly bonded together, and the electrodeseparation (alloy separation) is prevented from occurring.

In the solar cell having the above configuration according to theembodiment, as shown in FIG. 1-4, in the area B and the area C where thealuminum electrode 17 and the back-surface silver electrode 19 arepartially overlapped each other, bonding strength is increased at theinterface between the silicon substrate (p⁺ layer 14) and the aluminumelectrode 17 (the aluminum electrode 17 partially alloyed with theback-surface silver electrode 19), thereby better bonding is realized.As a result, it becomes possible to effectively prevent the electrodeseparation (alloy separation). Furthermore, it becomes possible toprevent the printing failure (faulty electrode fabrication) caused bythe difference in the thickness of the aluminum electrode 17 and theblack-surface silver electrode 19 after the firing in the electrodeforming process (a value obtained by subtracting the thickness of theback-surface silver electrode 19 from the thickness of the aluminumelectrode 17).

As described above, in the solar cell according to the embodiment, whilethe faulty electrode fabrication is prevented, the silicon substrate (p⁺layer 14) and the aluminum electrode 17 (the aluminum electrode 17partially alloyed with the back-surface silver electrode 19) are firmlybonded together, and thereby it is possible to effectively prevent theelectrode separation (alloy separation).

Next, a method of manufacturing the solar cell configured as aboveaccording to the embodiment is explained. Generally, in an inexpensivesolar cell a silicon substrate is used to generate photovoltaic powerwith a simple p-n junction and an n-layer of a few hundred nanometers inthickness is formed by diffusing group V elements such as phosphorous(P) on a p-type silicon substrate of a few hundreds micrometers inthickness. According to the present invention, both single crystal andpolycrystalline silicon can be used for the p-type silicon substrate;however, an explanation below is made with an example of a singlecrystal substrate with a plane orientation (100).

A process of producing the solar cell is briefly explained. In theprocess of producing the solar cell, the n-layer and a convex andconcave texture for keeping light on a substrate side therein areprovided on the surface of the p-type silicon substrate with a specificresistance of 0.1 to 5 ohm centimeters, and an antireflective coating isprovided thereon. Then, a silver electrode is provided on a frontsurface of the substrate.

Next, an aluminum electrode is provided on a back surface of thesubstrate, and a p⁺ layer is provided to create a back-surface field(BSF) effect so that electron concentration of the p⁺ layer is increasedby an electric field of a band structure and electrons in the p⁺ layerdo not disappear. With the aluminum electrode, it is also expected tocreate a back-surface reflection (BSR) effect of reflecting thelong-wave light that passes through the silicon substrate and reusingthe long-wave light to generate power. However, the aluminum electrodecauses considerable warpage of the substrate and induces substratecrack. Therefore, the aluminum electrode may be removed after the p⁺layer is formed by thermal treatment. Lastly, a silver electrode isprovided on the back surface of the substrate, and the solar cell isthereby completed.

In the following, the method of manufacturing the solar cell accordingto the embodiment is explained in detail with reference to the drawings.To produce the solar cell according to the embodiment, as shown in FIG.5-1, a p-type silicon substrate 11′ is sliced out of, for example, ap-type single-crystal silicon ingot produced by the pulling method or apolycrystalline silicon ingot produced by the casting method. Thesilicon substrate 11′ is etched by a thickness of about 10 to 20micrometers using, for example, a few to 20 wt/% of sodium hydroxide orsodium carbonate, and a damaged layer and dirt on the silicon surfacecaused by the slicing are removed.

Further, if necessary, the silicon substrate 11′ is washed with a mixedsolution of hydrochloric acid and hydrogen peroxide to remove heavymetals such as iron adhered onto the substrate surface. An anisotropicetching is then performed with a solution made by adding isopropylalcohol (IPA) to a similar low-concentrated alkaline solution to form atexture so that, for example, the surface of the silicon (11′) isexposed.

Next, an n-type diffusion layer 13 a is formed to form a p-n junction.In the process of forming the n-type diffusion layer 13 a, for example,phosphorus oxychloride (POCl₃) is used; a diffusion process is performedin a mixture gas atmosphere of nitrogen and oxygen at 800 to 900 degreesCelsius, and phosphorus is thermally diffused as shown in FIG. 5-2 toform the n-type diffusion layer 13 a with the inverse conductivity typeall over the surface of the silicon substrate 11′. The sheet resistanceof the n-type diffusion layer 13 a is, for example, several tens of (30to 80) ohm/square, and the depth of the n-type diffusion layer 13 a is,for example, about 0.3 to 0.5 micrometer.

To protect the n-type diffusion layer 13 a on the light receivingsurface, polymer resistive paste is printed by screen printing and driedto form resist. The n-type diffusion layer 13 a formed on the back andside of the silicon substrate 11′ is removed by soaking the siliconsubstrate 11′ in a solution of, for example, 20 wt/% potassium hydroxidefor a few minutes. The resist is then removed by an organic solvent toobtain the silicon substrate 11′ with the n-type diffusion layer 13formed all over the surface (light receiving surface) thereof as shownin FIG. 5-3.

As shown in FIG. 5-4, the antireflective coating 15 made of a siliconoxide film, a silicon nitride film, or titanium oxide film is formed onthe n-type diffusion layer 13 in a uniform thickness. In the case of thesilicon oxide film, for example, the antireflective coating 15 is formedby plasma chemical vapor deposition (CVD) using silane (SiH₄) gas andammonia (NH₃) gas as raw materials at a heating temperature equal to orhigher than 300 degrees Celsius under reduced pressure. For example, therefractive index is about 2.0 to 2.2, and the optimal thickness of theantireflective coating 15 is about 70 to 90 nanometers. It should benoted that the antireflective coating formed as above is an insulator;simply forming a front-surface electrode on the antireflective coatingdoes not produce a solar cell.

Next, an aluminum paste including a glass is printed all over the backsurface of the silicon substrate 11′ (an opposite surface of thelight-receiving surface) by screen printing and dried to form analuminum paste layer 17 a all over the back surface of the siliconsubstrate 11′ as shown in FIG. 5-5. On the aluminum paste layer 17 a areformed openings each corresponding to a part where the back-surfacesilver electrode 19 is formed. The thickness of the aluminum paste layer17 a can be adjusted, for example, by the wire diameter of a screen maskor emulsion thickness.

Next, a silver paste for the back-surface silver electrode 19 is printedby screen printing all over the back surface of the silicon substrate11′ (an opposite surface of the light-receiving surface), on which isformed the aluminum electrode 17, and dried to form a silver paste layer19 a as shown in FIG. 5-6. In the process of forming the aluminum pastelayer 17 a and the silver paste layer 19 a, the thickness of each layeris adjusted so that the thickness t_(Al) of the aluminum electrode 17 isto be larger than the thickness t_(Ag) of the back-surface silverelectrode 19 and a difference between the thickness t_(Al) of thealuminum electrode 17 and the thickness t_(Ag) of the back-surfacesilver electrode 19 is to be within a range from equal to or more than10 micrometers to equal to or less than 30 micrometers after the firing.

The silver paste can be printed by using a screen mask with a pattern ofemulsion 27 formed on a mesh 25 as shown in FIGS. 5-7 and 5-8. A maskframe 29 is formed at the periphery of the surface of the mesh 25opposite to the surface on which the emulsion 27 is present. Similarly,the aluminum paste can also be printed by using the screen mask with apattern of the emulsion 27 formed on the mesh 25. The thickness of thealuminum paste can be adjusted, for example, by the wire diameter of themesh 25 that forms the screen mask or emulsion thickness. Similarly, thethickness of the silver paste can also be adjusted, for example, by thewire diameter of the mesh 25 that forms the screen mask or the emulsionthickness.

Conventionally, there has been a priority on optimizing the amount ofthe aluminum paste to be coated that affects the amount of the warpageof the substrate or characteristics of the solar cell (aforementionedBSF effect or BSR effect). Therefore, the amount of the silver paste tobe coated has not been optimized, and the thicknesses of theback-surface aluminum electrode and the back-surface silver electrodeafter the firing have been almost the same.

Next, a silver paste for the front-surface silver electrode 21 isprinted by screen printing all over the front surface of the siliconsubstrate 11′ (light-receiving surface), on which is formed theantireflective coating 15, and dried to from a silver paste layer 21 aas shown in FIG. 5-9. The thickness of the silver paste can be adjustedby the wire diameter of the mesh 25 that forms the screen mask, emulsionthickness, or the like.

Next, in a firing process of forming the electrodes, the paste layersfor both the front-surface and back-surface electrodes are firedsimultaneously at about 600 to 900 degrees Celsius for a few to morethan 10 minutes. On the front-surface (light-receiving surface) of thesilicon substrate 11′, the silver paste layer is fired to form thefront-surface silver electrode 21 as shown in FIG. 5-10; while theantireflective coating 15 is melting, silver material contacts siliconin the silicon substrate 11′ through glass material included in thesilver paste, and the antireflective coating 15 is solidified again.Thereby, conductivity between the front-surface silver electrode 21 andthe silicon can be obtained. The above process is generally called afire-through process.

On the other hand, on the back surface (opposite surface of thelight-receiving surface) of the silicon substrate 11′, the aluminumpaste layer is fired to form the aluminum electrode 17 and the silverpaste layer is fired to form the back-surface silver electrode 19 asshown in FIG. 5-10. Then, the aluminum in the aluminum paste reacts tothe silicon in the silicon substrate 11′, which forms the p⁺ layer 14beneath the aluminum electrode 17. The layer is generally called aback-surface field (BSF) layer, and contributes to improve the energyconversion efficiency of the solar cell. In the silicon substrate 11′,an area between the n-type diffusion layer 13 and the p⁺ layer 14 ismade to the p-type layer 11.

The silver paste reacts directly to the silicon in the silicon substrate11′ in an area where the silver paste directly contacts the siliconsubstrate 11′, while, in an area where the silicon paste contacts thealuminum paste, three metals of silicon in the silicon substrate 11′,aluminum in the aluminum paste (aluminum electrode 17), and silver inthe back-surface silver electrode 19 are partially alloyed. The solarcell is completed by the solar cell manufacturing process involving theabove steps. In a module manufacturing process performed after the cellfabrication, a lead tab of copper is provided on the back-surface silverelectrode 19 to take out electric power.

According to the method of manufacturing the solar cell configured asabove of the embodiment, in the area B and the area C where the aluminumelectrode 17 and the back-surface silver electrode 19 are partiallyoverlapped each other, bonding strength is increased at the interfacebetween the silicon substrate (p⁺ layer 14) and the aluminum electrode17 (the aluminum electrode 17 partially alloyed with the back-surfacesilver electrode 19), thereby better bonding is realized. As a result,it becomes possible to effectively prevent the electrode separation(alloy separation). Furthermore, it becomes possible to prevent theprinting failure (faulty electrode fabrication) caused by the differencein the thickness of the aluminum electrode 17 and the back-surfacesilver electrode 19 after the firing (a value obtained by subtractingthe thickness of the back-surface silver electrode 19 from the thicknessof the aluminum electrode 17).

Therefore, according to the method of manufacturing the solar cellconfigured as above of the embodiment, it is possible to realize a solarcell in which, while faulty electrode fabrication is prevented, thesilicon substrate (p⁺ layer 14) and the aluminum electrode 17 (thealuminum electrode 17 partially alloyed with the back-surface silverelectrode 1-9) are firmly bonded together to effectively preventelectrode separation (alloy separation).

Second Embodiment

In the chapter of a second embodiment, the solar cell according toanother embodiment of the present invention is explained. In the firstembodiment described above, the thickness of the aluminum electrode 17is uniform; however, the thickness of the aluminum electrode 17 is notnecessarily uniform according to the present invention.

In the present invention, when, for example, the thickness of theback-surface silver electrode 19 is fixed, it is necessary to increasethe thickness of the aluminum electrode 17. In this case, consumption ofthe aluminum paste increases, resulting in high manufacturing costs. Inaddition, the thicker aluminum electrode 17 increases the warpage of thesubstrate caused by the stress generated by heating or cooling.

To prevent the above problems, it is effective to increase the thicknessof the aluminum electrode only in an overlapped area 31 of the aluminumelectrode 17 and the back-surface silver electrode 19 as shown in FIG.6, or to increase the thickness of the aluminum electrode only in anoverlapped area of the aluminum electrode 17 and the back-surface silverelectrode 19 with its neighboring area 33 as shown in FIG. 7. A basicstructure of the solar cell according to the embodiment shown in FIGS. 6and 7 is similar to that of the solar cell according to the firstembodiment described above except the overlapping of the aluminumelectrode 17 and the back-surface silver electrode 19, and therefore,reference may be had to the description of the first embodiment.

Fabrication of the above aluminum electrode 17 can be realized byincreasing the thickness of the emulsion of the screen mask for formingan electrode, which is described in the first embodiment, in aneighboring area of the overlapped area 31 of the aluminum electrode 17and the back-surface silver electrode 19, or in a neighboring area ofthe overlapped area of the aluminum electrode 17 and the back-surfacesilver electrode 19 with its neighboring area 33.

INDUSTRIAL APPLICABILITY

As described above, the solar cell according to the present invention isuseful as a solar cell configured with an aluminum electrode and asilver electrode for extracting electric power partially overlapped witheach other.

1-6. (canceled)
 7. A solar cell comprising: a photoelectric conversionsubstrate that includes a first surface and a second surface; a firstelectrode arranged on the first surface; a second electrode arranged onthe second surface; and a third electrode that extracts electric powerfrom the second electrode, and arranged on the second surface with aperiphery portion overlapping the second electrode in an in-planedirection of the photoelectric conversion substrate, wherein a thicknessof the second electrode is larger than a thickness of the thirdelectrode, and a difference between the thickness of the secondelectrode and the thickness of the third electrode is not less than 10micrometers and not more than 30 micrometers.
 8. The solar cellaccording to claim 7, wherein the thickness of the second electrode islarger than the thickness of the third electrode only in an area wherethe second electrode overlaps the third electrode.
 9. The solar cellaccording to claim 7, wherein the second electrode is an aluminumelectrode, and the third electrode is a silver electrode.
 10. A methodof manufacturing a solar cell that includes a photoelectric conversionsubstrate having a first surface and a second surface, the methodcomprising: forming a first electrode on the first surface; forming asecond electrode on the second surface; forming a third electrode thatextracts electric power from the second electrode on the second surfacewith a periphery portion overlapping the second electrode in an in-planedirection of the photoelectric conversion substrate, wherein a thicknessof the second electrode is larger than a thickness of the thirdelectrode, and a difference between the thickness of the secondelectrode and the thickness of the third electrode is not less than 10micrometers and not more than 30 micrometers.
 11. The method accordingto claim 10, wherein the thickness of the second electrode is largerthan the thickness of the third electrode only in an area where thesecond electrode overlaps the third electrode.
 12. The method accordingto claim 10, wherein the second electrode is an aluminum electrode, andthe third electrode is a silver electrode.